An alkaline storage battery that uses a negative electrode containing a hydrogen storage alloy as a negative electrode active material exhibits excellent output characteristics and also has high durability (e.g., superior life characteristics and/or storage characteristics). Accordingly, such an alkaline storage battery has been gaining attention as, for example, an alternative to a dry battery and a power source for an electric vehicle, and the like. Meanwhile, a lithium ion secondary battery has been used in similar applications in recent years. Therefore, from the viewpoint of highlighting the advantages of the alkaline storage battery, there is a need to further improve the battery characteristics such as the capacity, the output characteristics, and/or the life characteristics.
In general, hydrogen storage alloys contain an element having high affinity for hydrogen and an element having low affinity for hydrogen. As the hydrogen storage alloys, those having a crystal structure such as an AB5-type (e.g., CaCu5-type), an AB3-type (e.g., CeNi3-type), or an AB2-type (e.g., MgCu2-type) are used, for example. In these crystal structures, the element having high affinity for hydrogen tends to be located in an A-site, and the element having low affinity for hydrogen tends to be located in a B-site.
To improve the battery characteristics of the alkaline storage battery, attempts have been made to optimize the performance of hydrogen storage alloy powders.
For example, in PTL 1, a hydrogen storage alloy that contains Nd and has a ratio (B/A ratio) of an element having low affinity for hydrogen to an element having high affinity for hydrogen of 3.40 to 3.60 is combined with a hydrogen storage alloy that contains La and has a B/A ratio of 3.50 to 3.70.
In PTL 2, a hydrogen storage alloy having a B/A ratio of 2.5 to 4.5 is combined with a hydrogen storage alloy having a B/A ratio of 4.5 to 5.5 and contained in an amount of 50 mass % or less.
In PTL 3, a hydrogen storage alloy having a net-like continuous alloy skeleton structure in the matrix of the hydrogen storage alloy is investigated from the viewpoint of increasing the capacity, the life characteristics, and the like.